Cathode ray tube



Oct. 28, 1941. F. GRAY 2,260,313

CATHODE RAY TUBE Filed Feb. 8, 1939 FIG.

SIGN/4 L6 F/GZ F/G.3 H64 k i a a a w b ORDINARY u u TUBE 0F TUBE 0F CA 7/4005 RAY 5 Tl-IIS INVENTION THIS INVENTION TUBE 0F k l, BEFORE E AFTER PRIOR ART I! a COMPENSATION q; COMPENSATION MODULA TING VOLTA GE MODULA TING VOLTAGE MODULA TING VOLTAGE {FIG 5' HIGH SIGNAL VOL TA GE Low SIGNAL VOLTAGE /N VE N 7 0/? F. GPA V Maw, I

A T TORA V I Patented Oct. 28, 1941 Telephone -Labora-tories, Incorporated,

New I York; N. Y a corporation of New York Application February}, 1939,.Serial'No. 255,226 I IS'GlaimsL-f (01.25047) application relates to a cathode ray '7 tubes and more specifically to electron optical arrangements for said cathode ray tubes.

It is an object, of this invention to provide a.

novel method of and apparatus for modulating itacts very much, like thecontrol grid in an amplifier tube. The characteristic curve of beam current Versus modulating voltage in such a oath ode ray tubeis. similar to the characteristic curve of an amplifier tube; it is notlinear and tails off.

a o h n e end nb efmmm cat ode a.

slowly at itslower end. This lack oi -linearity distorts the reproduction of shades in a television image, and the tailing off at the lower end is particularly objectionablffor it results almost complete loss'of detail in the darker po ttions of an image.

It has been discovered to a large extent' 'be corrected by modulating the beam current byineans of an anode immediately adjacent the cathode. In such an arrangement the current drawn from the cathode ecrease fwith,theanodevoltage, and ceases abruptly at zero voltage so that there is no tailing on? inthe usual sense. "The';'curr'entdrawn from the oathofthe modulating voltage, but this lack oi"1ii earity is more or less corrected by introducing a" variation mine angular aperture of the electron lens system of this invention.

It is nother'object' of'this invention to provide a novel electron'lens systemin which modulatin voltages are applied "to a member Which/1S adode does; however, vary'as the three halves power jacent and positively biased with respect to the cathodeof the system.

In accomanCe w'ith'fthe invention; there is provided, way of ez'k'ainp'l'e, a cathode ray device comprising an evacuated envelope enclosing a novel electrode assembly and a screen or target. This assembly comprises, in order, going towards the-screen or target, an' equipotential cathode constituting a plane source of electrons, a lating-pr controlel'ement or anode adjacent the cathode, an' auxiliary apertur'ed electrodewhich isleplaced atfa' potential which is slightly negative with respect to the modulating anode, ajhigh potential tubular anodehaving'an' apertured' dia-i fihl a i'ngnear the end toward the Cathode, and

cylindrical 'final anode {which may comprise-a coating on the inside walls of the enclosure), and;

deflecting elements-to cause the beam ofv elec v trons to move over the surface of the screen .or target; The current density drawn from the;

cathode varies as thethree halves ;.power-of the modulating voltagebut thisinon-linearityiscom pensated'for by the-shifting in-thedivergence'of focus of the beam; which changein focuschanges the number-of electrons thatgpasses through the aperture in the diaphragmiat'the remote end of With proper dimensioning and spacing of the electrode-elements, thischange in divergence may beused to; correct'for the nonthe tubular anode.

linearity in the ;;current Idensitydrawn from the cathode, and the beam current made to vary al-vmost linearly with the modulating voltage.- With this method of modulation the current; can-be:

made to-cease abruptly at' zero potential.

The invention will be more readily understoool by ref erring to the following description taken in connection with the accompanying drawing forming a part thereof; in which: f Y

Fig 1. is a schematic 'diag'ram'of a cathode'r'ay tube' a'iid' some of its" associated circuits useidlto" illustrate the invention; and" grammatic representations used to' explain the operation of the invention.-

Re'fe'rring more particularly to.- the' drawing, Fig. 1 shows by way of examplel a'cathode ray; tubecontaining an electrode assembly making use of positive modulation. The cathode ray device comprises an evacuated envelope I10 mitsing a cathode H, a"mod ula ting or control anode |2, an auxiliary electrode 13, a;second anode it, a third anode comprisinga conducting-coating] 5 onthe insidevwalls of the enclosure II), a pair of deflecting plates l6, l6 and a fluorescent screen or target [81: Outside the container isa pairo'f deflectihgcoils IT, I7.

The cathode n has a flat end l9 which isonidej coated andconstitutes a plane, equipoteritiali source of electrons. After the end of the cathode is coated, a m'etal sleeve 20 is putover it mere-5:1 vent any new of current from the sidewallsiof the cathode H to the anode! 2. Thesleevelfl may be either flush with the end of the cathode; f or may project slightly beyond the end thereof to reduce still further anyfiow of current to the modulating or Control'anode 12. The cathode'is heated by a'heater'member 2| receiving current from the source'zz'. The modulating or control Figs. 2' to 6, inclusive, are'graphical and aa;

trode is placed at a potential which is not very far from zero (cathode) potential by means of the source 28. above zero or slightly below zero invariouscases depending on the geometry of the particular electron lens system.

The electrons pass through the aperture in ,th auxiliary electrode l3 into the high potential tubular anode l4 having two apertured diaphragms 29 and 30 at opposite ends thereof. This anode is placed at a high potential with respect to the cathode II by means of the source 3| through an inner tap 32.

As the electrons pass through the aperture in the diaphragm 30 they come into a region receiving its electric field from a high potential coating l5 of suitable conducting material such as, for example, aquadag, which receives its potential from source 3|. It is placed at a high positive potential with respect to the tubular anode I4. While the beam is in this region, it is caused to scan every elemental area of the fluorescent screen 18 in turn by means of deflecting signals respectively applied between the deflecting plates I6, 16' and passed through the deflecting coils l1, l1. Any suitable electrostatic and electromagnetic sweep circuits of the proper frequency may be used in connection with the deflecting plates and deflecting coils. If desired, two sets of plates or two sets of coils may be used instead of one set of each.

The electron lens action of the various electrode elements will now be described. When the modulating anode I2 has a positivepotential applied thereto, electrons are drawn from the cathode in the form of a diverging beam. (See Figs. 5 and 6 which show the approximate boundaries of the beam in ltS path through the electron lens, the region between the boundaries in Fig. 5 being filled with dotted lines to indicate greater beam density than in Fig. 6.) This beam tends to become converging as it passes through the aperture in the auxiliary electrode 13 due to the negative or retarding potential applied thereto with respect to the bias or average potential of the modulating anode l2. The convergence of the beam is further increased as it passes through the aperture in the diaphragm 29 and the beam comes to a focus at a point P somewhere between the diaphragms 29 and 30. Thus the combined lens system immediately in front of the cathode is equivalent to a single short focus lens; and, since the electrons leave the cathode normal to its surface, they are brought to focus in a small spot at P. An image of this spot is projected on the screen by the positive lens at the end of the tube I4 where it projects into the tubular or cylindrical anode formed by the coating IE on the inside of the glass envelope l0.

The initial electron velocities and various imperfections in the lenses spread the focal point P out along the axis, and for this reason only the central portion of the beam is used for projecting a small spot on the screen. The periphery of This potential may be slightly the beam is cut off by the apertured diaphragm 30 which also serves another purpose which will be considered below.

As the potential of the modulating anode I2 is lowered, the current density drawn from the cathode decreases and finally ceases abruptly with no tailing off, provided that the auxiliary anode I3 is at approximately zero potential, so that there is no appreciable penetration of electric fields through the aperture of the modulating anode [2. In order, however, that the lenses at the auxiliary electrode I3 be not too short in a n focal length, the plane of its aperture should be slightly positive with respect to the cathode.

Penetration of the field from the high potential anode l4 makes this plane more or less positive,

depending on the thickness of the auxiliary disc and the diameter of the aperture. To secure the most favorable voltage in the plane of the aperture, the auxiliary electrode I3 may require a small bias with respect to the cathode, and this bias may be either positive or negative depending on the dimensions of the electrode. the bias has an appreciable value, the beam may cut ofi at a voltage slightly different from zero. This is of no particular disadvantage in the operation of the tube.

' Referring now to Figs. 2, 3 and 4, three curves are shown of beam current versus modulating voltage. In Fig. 2 is shown the characteristic curve of the ordinary present-day cathode ray tube employing a Wehnelt cylinder biased negatively with respect to the cathode. It will be noted that this characteristic curve is similar to the characteristic curve of an amplifier tube; it is not linear and tails ofi slowly at its lower end, this lack of linearity distorting the reproductions of shades in a television image, and the tailing off at the lower end being particularly objectionable for it results in almost complete loss of detail in the darker portions of an image. Fig. 3 shows the characteristic curve of beam current versus modulating voltage that would result from positive modulation without any correction or compensating caused by the limiting action of the aperture in the diaphragm 30. This curve shows that the current density drawn from the cathode varies as the three halves power of the modulating voltage. This non-linearity, which would also appear in the beam current (assuming there is no correction at the apertured diaphragm 30) is more or less corrected by causing the motion of the focal point P to affect the number of electrons which pass out into the beam. As the modulating voltage is decreased, the focal length of the combined lens system increases and the focal point P moves towards the diaphragm 30 as shown in Figs. 5 and 6, Fig. 5 representing the situation where a relatively high positive signal voltage is applied to the anode l2 and Fig. 6 where the anode voltage is decreased to a relatively low value. The intensity of the beams in Figs. 5 and 6 is represented by the distance between the dotted lines representing the beam. As shown in Fig. 5, the electron beam leaves the point P with an increased divergence and thus a smaller percentage of the electrons in the anode' l4 passes out through the aperture in the diaphragm 30 than in the case of, the smaller beam intensity shown in Fig. 6. In Fig. 6 there are fewer electrons in the beam within the anode [4 .but' a greater percentage of them passes proper dimensioning and spacing of the elec- When ascetic samesense with'the variationsin'said potential, j

ait r'gence i can re -mace r a age I its from the cathode; and able to vary almost linearly" T various degrees of'correetion can fbe 'ob It will-be appreciated that the -'dimens'ions--of the electrode system are moreordess critical -al-- though s'omeof them mayvary within reasonracticalelectrode arrangement, the follow ing"d inensions 'aregiven: The diameter of the i I cathode ll'is' 'fi Qfaninch, the inside diameter correctflfor the non liriearityf-in v gjv'bltag'e as 'shown the} istrc' curve ofbeamcurrent-versus mod oltage irr-Figa 4. By proper propore' s an example of the dimensions of the modulating'ir'anode l2- and the tubular and the auxiliary electhe spacing between the mounting electrode 2 "and the auxiliary electrode lt is of an inch, the: spacing" between thfl auxiliary electrode I3 andthe diaphragm 1970f the anode fl 4" is of aninch; the spacing between the diaphragm'S'O and-the far end (from the cat'hode') of "the'tuJ'oular'anode I4 is 2 inches and-" the-distance of' the apertured dia- -ph' 3L0 fro'm'the'far end of the tubular anode j oi an inch; the aperture in the moduing l'ectrode'l2 is of an inch in diameter,

30 is ss'jof an-inch. The cap extends V The biason themodulating anode-l2 i about--*volts positive, the signal varying between"approximately 2 5 volts to +25 ts the potential 10f the auxiliaryieljectrode I3 "volts; the potential of the anode I4 de 1000' volts and the potential'oi the conating" I 5 is made about 3000 volts. If urrent fcompcnent is included in the 'ainjtief l2 may be varied depending *on' the value oi' thisdirejct cfurrent'component. If the cornpon'ent is large; theibias' may be omitted or even in e negative. The important thing is that lieclire'ctf'c'u'r'rent potential of the anode 12 be positive with respect tothe cathode.

aperturezm' the modulating electrode I3 is" 8 of; an inch, the aperture in'the diaphragm "of an inch and'theaperture in the dias'itiveflby "a small amount' which is signal, the bias" on the modulating cathodeand target and relativelycl'ose to the former, means' for varying the potentialeoflsaid conducting element relative to nthat v-orrsaid cathode, additionalmeans cooperating withsaid cathode and said element to cause the major portion of the emitted electrons to take the form of a beam which is diverging-when it reaches trode'fIZ-Qis-s of'an inch,- the'spacing between said plane and later' converginglto. a focus *on said axis'at a point dependentlupon the potential of said conducting element and also togcause the number of emitted electrons to var'y in the same sense withthe variations in said potential,

- said additional means comprising an electrode the maximum potential impressed upon said con-' ducting element, and means includinganapen. tured diaphragm in saidanode'for utilizing the change in position of 'focus to vary thernumber of electrons reaching said target. V r

3. A cathode ray tube device comprising an electron emitting cathode, a targetga conducting element having an annular portion symmetrically encircling the tube axis which passes through. the center of the emitting portiohofthe cathode and extends tothe target, said annular portionexq tending on the side towards said target to a plane perpendicular to said axis between said cathode and target and relatively close to the former, means for varying the "potentialof .said conducting element relative -to that of said cathode, additional means cooperating with said mv i w j m ifi ati ns may be'made in the I e bodiment of the-invention "as above disclosed, the soon heinvention'being indicatedby the appended c anns. v TWhat is laimed' is:

' 15 A cathode ray tube device comprising an electron emitting cathode, a target, a conducting e'le'ment having an annular portion symmetrically-encircling the tube axis which passes through thecenter of the'emitting portion of the cathodeandextends tothe target, said annular portion extending on the side towards said target to'a plane perpendicular to said axis between said'cathodeand target and relatively'close to the" former, means for varying the potential of s'aidc'onductihg element relative to-that-of said cathode, additional means'cooperating' with said cathode and said element to cause'th'e major portion of the emitted electrons totake the'i'orm era beam which is diverging when it reaches said plane and later converging to a focus on said'axis at a point dependent upon the potenn51 "bf-said conducting elementand also to cause ,tiie mmser cremated electrons to var in ale cathode and said element tolcause the major portion of the emitted electrons to take theform of a beam which is divergingwhenlit reaches said plane and later convergingto al focus/on said axis at a point dependent upon thepotential of 'saidelement and also to cause the number of emitted electrons to vary in the same sense with the variations in said potential, said additional means comprising an electrode assembly top'rov id'e an electron retarding-field in the spacing into which saiddiverging beam is projected, and means for utilizing the change in position of focus to vary'the number of electrons reaching said target. 41A cathode ray tube device ccmprisingzan electron emitting cathode, 'a target, aconducting element having an annular portion symmetrically encircling the' tube axis which' passes throughfthe center of the emitting-portion of the 'cathodezi'and extends to fthe target, said annular portion extending-on the side towards said-target to a plane perpendicular to said axis between said cathode and target and relatively close to theformer, means for varying the potential of said. can ducting-element relative to thatof said cathode, additional means cooperating with said cathodeand said element to cause the major portion o-f V the eiriitted- 'electrons toQtake the form 'ofabe'am and later converging to a focus on said axis at a point dependent upon the potential of said element and also to cause the number of emitted electrons to vary in the same sense with the variations in said potential, and an electrode having a small aperture in the path of said beam, the dimensions of said aperture relative to those of the different cross-sections of said beam in the plane of the aperture as the focal point moves being such that the number of electrons passing through said aperture will vary with variations in the position of said focal point.

5. A cathode ray tube device comprising an electron emitting cathode, a target, a conducting element having an annular portion symmetrically encircling the tube axis which passes through the center of the emitting portion of the cathode and extends to the target, said annular portion extending on the side towards said target to a plane perpendicular to said axis between said cathode and target and relatively close to the former, means for biasing said conducting element to a potential which is positive with respect to that of said cathode, means for varying the potential of said conducting element relative to that of said cathode, and electrode means positioned adjacent said conducting element and on the side thereof toward said target and effectively surrounding a line connecting the center of said cathode with that of the target and symmetrical therewith and cooperating with said conducting element to set up a field adjacent the latter to cause, in cooperation with said cathode and said element by reason of the field therebetween, the emission of electrons from said cathode to cease when the potential of said conducting element is reduced to a value which is at least substantially equal to that of said cathode.

6. In a cathode ray device, a cathode, a target, means for creating a diverging electrostatic field near said cathode, means for varying the strength of said field in accordance With signals, and means for creating a converging field in a region farther along the axis of the device for forming the electrons of said cathode into substantially a point, said point moving along the axis of said device as the strength of said diverging field is varied, and means including an apertured diaphragm for variably intercepting portions of said beam in accordance with the signals applied to said diverging field creating means, said diaphragm being between said focal point and said target.

'7. In a cathode ray device, an evacuated envelope enclosing a cathode, an anode adjacent said cathode, a second anode, means for biasing said first anode to a positive potential with respect to said cathode, means for placing the second anode at a much higher positive potential than said first anode, means for shielding said cathode from the potential of said second anode, means for applying modulating voltage between said first anode and said cathode from a range of voltages such that the number of electrons emitted from said cathode is varied in accordance with some positive power of the modulating voltage, the positive bias on said first anode being of such value as to ensure this action.

8. In a cathode ray device, a cathode, an anode adjacent said cathode, a second anode, means for biasing said first anode to a positive potential with respect to said cathode, means for placing said second anode at a much higher positive potential than said first anode, means for shielding said cathode from the potential of said second anode, means for applying modulating voltage between said first anode and said cathode from a range of voltages such that the number of electrons emitted from said cathode is increased with increasein modulating voltage, the, positive bias of said first anode being of such value as to ensure this action, and means including an apertured diaphragm in said second anode for variably intercepting portions of said beam in accordance with the intensity of the modulating voltage.

9. In a cathode ray device, a cathode, a target, means for creating a diverging electrostatic field near said cathode, means for varying the strength of said field in accordance with signals, and means for creating a converging field in a region farther along the axis of the device for forming the electrons of said cathode into substantially a point, saidpoint moving along the axis of said device as the strength of said diverging field is varied, means including an apertured diaphragm between said focal point and said target for variably intercepting portionsOf said beam in accordance with the signals applied to said diverging field creating means, and means for focussing an electron image of said aperture on said screen.

10. An electron gun arrangement comprising an equi-potential cathode having one end oxide coated, a cap around a portion of said cathode, said cap projecting slightly beyond the oxide coated end of said cathode, an apertured diaphragm parallel to and closely adjacent the oxide coated end of said cathode, a second apertured diaphragm parallel to said first apertured diaphragm and farther removed from said cathode than said first diaphragm, a tubular anode symmetrically arranged with respect to and surrounding a portion of the axis of said gun farther remote from said cathode than said second apertured diaphragm, said tubular member comprising two apertured diaphragms, a metallic cylindrical member symmetrically arranged with respect to said axis and generally surrounding a portion of said axis the center of which is farther removed from the cathode than the center of the portion of the axis surrounded by said tubular anode, means for placing said first apertured diaphragm at a positive potential with respect to said cathode, means for placing said second apertured diaphragm at a potential which is approximately that of said cathode but which is negative with respect to the potential of said first apertured diaphragm, means for placing said tubular anode at a potential which is highly positive with respect to said cathode and said first apertured diaphragm, means for placing said cylindrical member at a potential which is highly positive with respect to that of said tubular anode, and means for applying modulating voltages between said first apertured diaphragm and said cathode.

11. An electron gun comprising a cathode for generating a stream of electrons, a modulating anode adjacent said cathode, an auxiliary electrode adjacent said anode, a tubular anode having an apertured diaphragm therein in the portion thereof remote from said cathode, said anodes and said auxiliary electrode cooperating to focus the electrons to a point on the axis of said electron gun near the plane of said apertured diaphragm, and means for applying signals between said cathode and modulating anode to simultaneously cause the numberof emitted electrons to vary in the same sense as the variations in potential of said signals and change the focus of said electrons with respect to said diaphragm.

12. An electron gun comprising a cathode for generating a stream of electrons, a positively biased modulating anode adjacent said cathode, means for applying signals to said modulating anode to vary the emission from said cathode as the three halves power of said modulating volt- .age, and means for correcting in part at least for the departure from linearity in the characteristic curve of emission from the cathode Versus modulating voltage.

, 13. The method of controlling electrons emitted from a cathode which comprises the steps of varying the number of electrons emitted by the cathode in accordance with a varying voltage, thenumber emitted increasing substantially as the three halves power of the varying voltage,

- 'forming the major portion, at least, of the emitted electrons into a beam, and selectively utilizing a portion only of said beam, said portion increasing with increase of said varying voltage but at a different rate which causes the selected beam to have an intensity which varies more nearly directly proportionally to the variations in said voltage than does the number of electrons in said beam prior to said selection.

'14. A cathode ray tube device comprising an electron emitting cathode, a target, a conducting element having an annular portion symmetrically encircling the tube axis which passes through the center of the emitting portion of the cathode and extends to the target, said annular portion extending on the side towards said target to a plane perpendicular to said axis between said cathode and target and relatively close to the former, means for biasing said conducting element to a potential which is positive with respect to that of said cathode, means for applying modulating voltage to said conducting element line connecting the center of said cathode with that of the target and symmetrical therewith for substantially equal to that of said cathode, said electrode means comprising an electrode element which is placed at a potential within a range having as a lower limit a potential which is slightly negative with respect to that of said cathode and having as an upper limit a potential which is approximately equal to that of the conducting element when only the biasing voltage is applied thereto.

15. A cathode ray tube device comprising an electron emitting cathode, a target, a conducting element having an annular portion symmetrically encircling the tube axis which passes through the center of the emitting portion of the cathode and extends to the target, said annular portion extending on the side towards said target to a plane perpendicular to said axis between said cathode and target and relatively close to the former, means for biasing said conducting element to a potential which is positive with respect to the potential of said cathode, means for varying the potential of said conducting element relative to that of said cathode, and electrode means effectively surrounding :a line connecting the center of said cathode to that of the target and symmetrical therewith and cooperating with said conducting element to set up a field adjacent the latter to cause, in cooperation with said cathode and said element by reason of the field therebetween, the emission of electrons from said cathode to cease when the potential of said conducting element is reduced to a value which is substantially equal to that of said cathode, said electrode means comprising a first additional electrode element adjacent said conducting element and a second additional electrode element adjacent said first additional electrode element and between said first additional electrode element and said target, said additional electrode elements cooperating with said cathode and said conducting element to form a beam of electrons which converges toward a point on the side of said first additional electrode element remote from said cathode.

FRANK GRAY. 

